Water-stable aggregation (WSA) is a basic process contributing to the maintenance of soil structure and the protection of soil organic matter. Crop residues incorporation in soil contributes to WSA mainly through microbially-mediated processes. The interactive effects of crop residue quality and mineral nitrogen, and the microbial agents responsible of these effects still remain a subject of debate. We designed an incubation experiment with the aim of clarifying some of these effects.

We incubated two high-C/N crop residues (wheat, C/N = 125.6; miscanthus, C/N = 311.3) applied at 4g C kg^-1dry soil with or without a mineral N supply (120 mg N kg^-1 dry soil), under controlled conditions (25°C and 217 g kg^-1 dry soil water content). Soil WSA, mineral N content, respiration, total microbial biomass, bacterial and fungal genetic structure, and hot-water polysaccharide content were measured during a 56-day incubation.

Following crop residue addition, WSA dynamics followed two distinct phases. During the first week, WSA was controlled by residue quality with higher values with wheat straw. In the remainder of the incubation mineral N levels was the main determinant with much higher WSA values in its absence. Early changes in WSA were related to changes in the total microbial biomass induced by the residue quality. Thereafter, the dynamics of WSA was more associated with the dynamics of hot-water extractable polysaccharide and greatly influenced by mineral N addition. Mineralization and microbial analyses suggested that mineral N input greatly stimulated early microbial decomposition activity through the transient stimulation of an opportunistic bacterial population which consumed labile binding agents. To the contrary, microbial polysaccharide production was high when no mineral N was added which led to the higher WSA in the late stage of decomposition in this treatment. The late stage of decomposition was associated with a particular fungal community also influenced by the mineral N treatment which suggests that the specific structure of the microbial community plays a greater role in the late stage than during the initial stages of WSA dynamics. Microorganisms play a dual role in controlling WSA dynamics by being both producers as well as degraders of aggregate binding agents, depending largely on the availability of mineral N.